This looks like tar, which would have to come from far away tarpits.

That was my first thought, and my second was that such is maybe just a trope.

Faraway tarpits means effort, expense, and inconvenience. Plant-sap pitch might be more likely as something with a double-use for sealing various castle cracks (wood, stone).

There's also the fact that pouring boiling tar or pitch would be difficult all the way around-- to heat as necessary, pour, and later to clean up. Whereas oil would be easier all-around, methinks. And much better if it could be locally-sourced.